Use of a thermoset binder for 3d printing of cemented carbide or cermet bodies

ABSTRACT

The use of a water-soluble thermoset binder for binder jetting of a cemented carbide or cermet green body is provided. The water-soluble thermoset binder includes a compound A, being at least one organic, non-aromatic substance, including at least two hydroxyl groups and a compound B, being at least one organic, non-aromatic substance, including at least two carboxyl groups, wherein the compound A and compound B are monomers or oligomers. The binder will lead to an increased strength of the printed green body.

The present invention relates to the use of a water-soluble thermosetbinder for binder jetting of a cemented carbide and cermet green body.The water-soluble thermoset binder comprises a compound A being at leastone organic, non-aromatic substance, comprising at least two hydroxylgroups and a compound B being at least one organic, non-aromaticsubstance, comprising at least two carboxyl groups, wherein the compoundA and compound B are monomers or oligomers.

BACKGROUND

Additive manufacturing, also known as 3D printing, generally involvesprinting and articulate one layer at a time using specialized systems.In particular, a layer of material may be deposited on the workingsurface of a build chamber and bonded with another layer of the same ora different material. Additive manufacturing may be used to manufacturearticles from computer-aided design models using techniques such as, butnot limited to, powder bed fusion (PBF), direct metal laser sintering(DMLS) or binder jet 3D printing.

Additive manufacturing of cemented carbide or cermet bodies arepreferably done by a 3D printing technique that first creates a greenbody after which the green body is sintered in a separate furnace.Binder jetting is the most common type of such 3D printing technique.

When printing cemented carbide and cermet bodies using binder jetting,the strength of the printed body, i.e. the green strength, must beenough to avoid breakage of the body during the subsequent handling,like e.g. depowdering, moving the bodies to a sintering furnace etc.

Thermoset binders, i.e. a chemical binder that reacts at an elevatedtemperature, are known in binder jetting of powders. In someapplications one component is added together with the powder whereas theother component is added during printing after which the printed body iscured. For cemented carbide and cermet powders this is disadvantageoussince one of the main issues is porosity in the sintered bodies. Addingone component to the powder decreases the powder-powder contact andcould lead to more porosity, or binder rich areas in the sintered state.

Thermoset binders containing different types of polymers are the mostcommon. However, due to solubility, the concentration of the polymersmust be kept low and large amounts of solvent is added during printingwhich can lead to problems when drying the printed body.

There are also epoxy binders, but they are usually neither environmentalnor health-friendly.

Another type of binders for use in binder jetting are binders that doesnot react chemically during curing but instead, polymers are entangledand thus increase the green strength. These binders can, if wetdepowdering is used, be dissolved by the cleaning fluid and the greenbody will then lose its strength.

US2019/0054527 describes a binder for binder jetting of e.g. ceramicscomprising two different polymers that will, during curing,non-covalently, couple the two different types of polymers.

One object of the present invention is to obtain a binder that is lesshazardous.

Another object of the invention is to obtain a binder for binder jettingthat can be added in a higher concentration, i.e. with less solvent.

Another object of the invention is to obtain a binder for binder jettingthat lead to an increased green strength.

Another object of the invention is to obtain a binder for binder jettingthat does not dissolve after curing, i.e. during depowdering etc.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to the use of a water-soluble thermosetbinder for binder jetting of a cemented carbide or cermet green body.The water-soluble thermoset binder comprises a compound A being at leastone organic, non-aromatic substance, comprising at least two hydroxylgroups and a compound B being at least one organic, non-aromaticsubstance, comprising at least two carboxyl groups, wherein the compoundA and compound B are monomers or oligomers with a molecular weight lessthan 2000 g/mol.

By thermoset binder is herein meant that the components in the binderwill harden by curing. Curing is usually induced by heating.

The thermoset binder comprises a compound A and a compound B andpossibly also other additives such as pH adjusters, surfactants etc. Thecompounds A and B should have low, or no reaction rate at roomtemperature to achieve a reasonable shelf life.

Compound A and B are both water soluble. By water soluble is hereinmeant that the compounds A and B described herein are soluble in waterin the amounts given below. Both compound A and B are monomers oroligomers. By oligomer is herein meant that that a compound with amolecular weight below 2000 g/mol, preferably less than 1000 g/mol.

The compounds A and B are also non-aromatic substances which is hereinmeant that compounds A and B are free from aromatic groups such asbenzyl groups, phenyl groups etc. Aromatic substances are unwanted sinceit can form harmful byproducts during the debinding and/or sintering.

Compound A and B will, after the green body is subjected to an elevatedtemperature during curing, react by cross linking and formingester-bonds to form a green body with high strength

Compound A comprises at least one organic, non-aromatic, substancecomprising at least two hydroxyl groups and where compound A is amonomer or an oligomer. Compound A can either be constituted by only onesingle organic, non-aromatic substance comprising at least two hydroxylgroups or being a mixture of two or more organic, non-aromatic,substance comprising at least two hydroxyl groups.

In one embodiment of the present invention, the at least one organic,non-aromatic, substance, comprising at least two hydroxyl groups doesnot contain an epoxy-group.

In one embodiment of the present invention, the at least one organic,non-aromatic, substance, comprising at least two hydroxyl groups doesnot contain any nitrogen, phosphorous, silicon, potassium, sodium orfluorine.

In one embodiment of the present invention, compound A is a polyol likee.g. a diol, triol or a tetriol, more preferably compound A is selectedfrom the group propylene glycol, glycerol, maltodextrin, erythritol,xylitol, sorbitol and mannitol, most preferably compound A is selectedfrom glycerol and maltodextrin.

Compound B comprises at least one organic, non-aromatic, substance,comprising at least two carboxyl groups and is a monomer or an oligomer.Compound B can either be constituted by only one single organic,non-aromatic substance comprising at least two carboxyl groups or beinga mixture of two or more organic, non-aromatic, substance comprising atleast two carboxyl groups.

In one embodiment of the present invention, the at least one organic,non-aromatic, substance, comprising at least two carboxyl groups doesnot contain an epoxy-group.

In one embodiment of the present invention, the at least one organic,non-aromatic, substance, comprising at least two carboxyl groups doesnot contain any nitrogen, phosphorous, silicon, potassium, sodium orfluorine.

In one embodiment of the present invention, the at least one organic,non-aromatic substance, comprising at least two carboxyl groups isselected from the group citric acid, tartaric acid, succinic acid,fumaric acid, maleic acid and carballylic acid, polyacrylic acidoligomer, most preferably compound B is citric acid.

The amount of compound A and B are such that the molar ratio of thehydroxyl groups in compound A and the carboxyl groups in compound B isbetween 0.1 and 10, preferably between 0.5 and 2.0, most preferably 0.8and 1.2.

In one embodiment of the present invention, the total amount of compoundA and compound B is between 2 and 80 wt %, preferably between 30 and 80wt % and most preferably between 30 and 50 wt %.

In one embodiment of the present invention, the total amount of compoundA and B is more than 80 wt % of the “dry” components, i.e. when thesolvent is excluded.

The thermoset binder can also contain other additives such assurfactants, pH-adjusters e.g. ethanol, ethylene glycol, ethylene glycolmonobutyl ether.

The compound A and B together with the, if added, other additives aredissolved in a solvent. Preferably, the solvent is water based, i.e. thesolvent contains more than 50 vol % water, preferably more than 60 vol %water, most preferably, the solvent is 100% water.

In one embodiment of the present invention, the binder comprisesglycerol in an amount of between 35 and 50 wt %, citric acid in anamount of between 15 and 25 wt %.

In one embodiment of the present invention, the binder can be usedtogether with other binders to further increase the green strength. Thetotal amount of compound A and B should then be at least 2 wt % of thetotal amount of binder.

The present invention also relates to a method of making a cementedcarbide or cermet body using a binder according to the above. The methodcomprises the following steps:

-   providing a ready-to-print cemented carbide or cermet powder,-   providing a water soluble thermoset binder,-   printing said powder by binder jetting technique into a green body,-   curing said body by heating at a temperature between 100° C. and    250° C.-   sinter said green body-   wherein the water soluble two-component binder comprises a compound    A being at least one organic, non-aromatic substance, comprising at    least two hydroxyl groups and a compound B being at least one    organic, non-aromatic ,substance, comprising at least two carboxyl    groups, wherein the compound A and compound B are monomers or    oligomers.

The term “cermet” is herein intended to denote a material comprisinghard constituents in a metallic binder phase, wherein the hardconstituents comprise carbides or carbonitrides of one or more of Ta,Ti, Nb, Cr, Hf, V, Mo and Zr, such as TiN, TiC and/or TiCN.

The term “cemented carbide” is herein intended to denote a materialcomprising hard constituents in a metallic binder phase, wherein thehard constituents comprise at least 50 wt % WC grains. The hardconstituents can also comprise carbides or carbonitrides of one or moreof Ta, Ti, Nb, Cr, Hf, V, Mo and Zr, such as TiN, TiC and/or TiCN.

The metallic binder phase in the cermet or in the cemented carbide is ametal or a metallic alloy, and the metal can for example be selectedfrom Cr, Mo, Fe, Co or Ni alone or in any combination. Preferably themetallic binder phase comprises a combination of Co, Ni and Fe, acombination of Co and Ni, or only Co. The metallic binder phase cancomprise other suitable metals as known to the skilled person. Theaverage content of metallic binder phase in the powder is 4-15 wt %,preferably 8-13 wt % or 10-13 wt %.

The ready-to-print powder used in the method of making a threedimensional printing of a cermet or cemented carbide body, can be anypowder comprising cermet or cemented carbide particles suitable forthree dimensional printing.

By cermet or cemented carbide particles is herein meant that theparticles have been pre-sintered. By pre-sintered it is herein meantthat the powder has been sintered, such as using solid state sinteringand/or liquid phase sintering.

Preferably, the ready-to-print powder is a pre-sintered powdercomprising spherical cermet or cemented carbide particles with goodflowability.

In one embodiment of the present invention the D50 of the cementedcarbide and/or cermet particles is 5-35 μm, preferably 10-30 μm, morepreferably 15-25 μm, most preferably 17-21 μm.

In one embodiment of the present invention, the ready-to-print powderhas been pre-sintered so that the porosity of the cermet or cementedcarbide particles is between 0-40% porosity, preferably between 5-20 vol%, or 15-20%.

In one embodiment of the present invention, the ready-to-print powderhas been pre-sintered so that the porosity of the cermet or cementedcarbide particles is between 0-20% porosity, preferably between 0-10 vol%, or 0-5 vol %, or that the particles are fully dense.

A certain porosity can in some cases contribute to the sinteringactivity during the sintering of the printed green body and depending onhow much sintering activity that is desired for a particular cermet orcemented carbide composition, the porosity can be adjusted.

The porosity can for example be measured in a LOM at 1000×magnification.

In one embodiment of the present invention, the ready-to-print powder ismade by:

-   mixing a cermet or cemented carbide raw powder and organic binder-   spray drying said raw powder and thereby form a granulated raw    powder-   pre-sintering said spray dried raw powder removing said organic    binder and thereby—form a pre-sintered granulated powder,-   preprocessing said pre-sintered granulated powder until the desired    particle size distribution is achieved and thereby form a    ready-to-print-powder.

By preprocessing is herein meant that the pre-sintered granulated powderis de-agglomerated since the granules can be joined in clusters whichcan cause problems during printing. This can be done by a gentle millingstep. If the grain size needs further adjusting, the milling step can beprolonged.

In one embodiment of the present invention the spray dried powder issieved before the pre-sintering step, preferably sieved to removeparticles larger than 42 μm in diameter. This is advantageous in that itreduces the risk of problems with very large particles in the powder.

In one embodiment of the present invention the ready-to-print powdercomprises cemented carbide particles. The cemented carbide particlescomprise WC with an average grain size of 0.5-5 μm or 0.5-2 μm.Preferably more than 80 wt % of the hard constituents are WC.

In one embodiment of the present invention the ready-to-print powdercomprises cemented carbide particles comprises WC and Co in an amount ofbetween 10 and 13 wt %. The cemented carbide particles have a D50 ofbetween 17 and 21 μm and a porosity between 0 and 5 vol %.

Binder jetting is a well known, relatively cheap, 3D printing method.The method comprises the steps of first applying a powder layer and ontosaid powder layer selectively depositing a binder, and then, layer bylayer, building a green body with the desired shape.

Curing is normally performed after the printing step. The thermosetbinder is cured whereby the green body gets a sufficient green strength.The curing can be performed by subjecting the printed green body to anincreased temperature of between 100 and 250 ° C., preferably between140° C. and 200° C. for time period between 1 and 100 hours, preferablybetween 1 and 5 hours, before removal of the excessive powder. In oneembodiment the curing is performed in a non-oxidation environment suchas in Ar or N₂ and/or in low pressure or vacuum.

After the curing step, all excess powder is removed from the green bodythrough a depowdering step. The depowdering can either be a dry process,i.e. the excess powder is e.g .blown of brushed off the green body. Thedepowdering can also be a wet process, i.e. immersing the green bodyinto a solvent of any kind and simply rinse of the excess powder. Alsovibrations such as ultrasonics etc. can be used, both in dry or wetdepowdering.

The green body is then sintered in a sintering furnace by liquid phasesintering. Liquid phase sintering is performed at a temperature abovethe temperature for when the metallic binder in a particular cermet orcemented carbide composition melts. Preferably, the liquid phasesintering takes place at a temperature between 1350° C. and 1500° C.preferably, the duration for the liquid phase sintering is between 30and 300 minutes, more preferably between 30 and 120 minutes.

The liquid phase sintering step is preferably performed in vacuum, i.e.so-called vacuum sintering. By that is herein meant that the pressure isbelow 0.5 mbar.

Liquid phase sintering step is common in the art of sintering cermetsand cemented carbides and usually contains a debinding step prior toreaching the liquid phase sintering temperature, usually at atemperature of between 200° C. and 550° C. for about 30 and 120 minutes.The debinding step is performed in order to remove any residual binder,e.g. thermoset printing binder.

In one embodiment of the present invention, the cemented carbide orcermet bodies made according to this invention are cutting tools such asan insert, drill or end mill.

In one embodiment of the present invention, the cemented carbide orcermet bodies made according to this invention can be wear parts such asnozzles, seal rings, pump impellers, flow controllers for oil/gas etc.

EXAMPLE 1

Green bodies in shape of rectangular blocks of 60×15×5 mm were printedfrom a pre-sintered cemented carbide powder with mostly spherical-shapedgranules that have a D10 of 11.1 μm, D50 of 20.3 μm and D90 of 33.6 μm.The composition of the powder was 12 wt % Co and balance WC. The averageporosity of the granules is 0 vol %, or very close to zero.

The binder was prepared by mixing water, ethanol, glycerol and citricacid. The components were thoroughly stirred until all components hadbeen dissolved. The composition is shown in Table 1. The molar ratio inTable 1 is thee molar ratio of the hydroxyl groups in compound A to thecarboxyl groups in compound B.

TABLE 1 Glycerol Citric Acid Molar Water Ethanol (Compound A) (CompoundB) ratio Invention 1 21.6 wt % 17.3 wt % 42.7 wt % 18.4 wt % 4.8/1

Printing was performed in a binder jetting printing machine with a layerthickness during printing of 50 μm. The printing of powder was done in a“ExOne Innovent+”. Saturation during printing was 110% and the scalefactor of the parts were set to 1.

The saturation of printing binder is defined as the percent of the voidvolume that is filled with printing binder at a specified powder packingdensity (here the powder packing density is set to 60%). A highersaturation is needed when printing with a powder comprising a largerfraction of porous particles as compared to a lower fraction of porousparticles.

During the printing the sequence for each layer was as follows: a 50 μmlayer of the powder was spread over the bed, printing binder was spreadin a pattern as defined in a CAD model, followed by drying of theprinting binder to remove all or some of the solvent of the printingbinder. This was repeated until the full height of the green body wasprinted. Thereafter curing was done overnight at 200° C. in vacuum.Depowdering was done manually by brush and pressurized air.

EXAMPLE 2

Green bodies were printed using the same process and cemented carbidepowder as described in example 1 with difference that the binder wasprepared by mixing Maltodextrin (dextrose equivalent around 18) andCitric acid with a commercial Aqueous Binder and that the saturationduring printing was 100%. The components were thoroughly stirred untilall Maltodextrin and Citric Acid had been dissolved. The composition isshown in Table 2. The molar ratio in Table 2 is thee molar ratio of thehydroxyl groups in compound A to the carboxyl groups in compound B.

TABLE 2 Competitor Aqueous Maltodextrin Citric Acid Molar Binder(Compound A) (Compound B) ratio Invention 3 96 wt % 2 wt % 2 wt % 1.23/1

For comparison, a green body using only the commercial Aqueous binderwas printed with the same powder and the same process as in Example 2.

EXAMPLE 3

After depowdering the strength of the green bodies were tested bytransversal flexural test with a speed of 10 mm/min and a preload of0.05N. The results are shown in table 3.

TABLE 3 Flexural Strength [MPa] Invention 1 4.61 Invention 2 2.55Comparative 1 1.65

In Table 3 it can clearly be seen that the green bodies with the binderaccording to the invention, Invention 1 and 2, shows a significantimprovement in green strength compared to the commercial Aqueous binder.

1. A use of a water-soluble thermoset binder for binder jetting of acemented carbide or cermet green body wherein the water-solublethermoset binder comprises a compound A being at least one organic,non-aromatic substance, comprising at least two hydroxyl groups and acompound B being at least one organic, non-aromatic substance,comprising at least two carboxyl groups, wherein the compound A andcompound B are monomers or oligomers with a molecular weight less than2000 g/mol.
 2. The use of the water-soluble thermoset binder accordingto claim h wherein compound A and B do not contain any epoxy group. 3.The use of the water-soluble thermoset binder according to claim 1,wherein compound A and B do not contain nitrogen, phosphorous, silicon,potassium, sodium or fluorine.
 4. The use of the water-soluble thermosetbinder according to claim 1, wherein the amounts of compound A and B aresuch that the molar ratio of the hydroxyl groups in compound A to thecarboxyl groups in compound B is between 0.1 and
 10. 5. The use of thewater-soluble thermoset binder according to claim 1, wherein compound Ais selected from propylene glycol, glycerol, maltodextrin, erythritol,xylitol, sorbitol and mannitol.
 6. The use of the water-solublethermoset binder according to claim 1, wherein compound B is selectedfrom citric acid, tartaric acid, succinic acid, fumaric acid, maleicacid, polyacrylic acid oligomer and carballylic acid.
 7. The use of thewater-soluble thermoset binder according to claim 1, wherein the totalamount of compound A and B is between 30 and 80 wt %:
 8. The use of thewater-soluble thermoset binder according to any of the preceding claimsclaim 1, wherein compound A is selected form glycerol or maltodextrinand compound B is citric acid.
 9. The use of the water-soluble thermosetbinder according to claim 1, wherein the binder includes glycerol in anamount of between 35 and 50 wt %, and citric acid in an amount ofbetween 15 and 25 wt %.
 10. A method of making a cemented carbide orcermet body comprising the steps of: providing a ready-to-print powderbeing a cemented carbide powder or cermet powder; providing a watersoluble thermoset binder; printing said powder by binder jettingtechnique into a green body; curing said body by heating at atemperature between 100° C. and 250° C.; and sintering said green body,wherein the water soluble thermoset binder comprises a compound A beingat least one organic, non-aromatic substance, comprising at least twohydroxyl groups and a compound B being at least one organic,non-aromatic, substance, comprising at least two carboxyl groups,wherein the compound A and compound B are monomers or oligomers.
 11. Themethod according to claim 10, wherein the ready-to-print powder is acemented carbide powder with a metallic binder in an amount of between10 and 13 wt % and wherein the carbide particles have a D50 of between17 and 21 μm and a porosity between 0 and 5 vol %.
 12. The methodaccording to claim 10, wherein compound A is selected from propyleneglycol, glycerol, maltodextrin, erythritol, xylitol, sorbitol andmannitol.
 13. The method according to claim 10, wherein compound B iscitric acid, tartaric acid, succinic acid, fumaric acid, maleic acid,polyacrylic acid oligomer and carballylic acid.
 14. The method accordingto claim 10, wherein the amounts of compound A and B are such that themolar ratio of the hydroxyl groups in compound A to the carboxyl groupsin compound B is between 0.1 and
 10. 15. The method according to claim10, wherein compound A is selected form glycerol or maltodextrin andcompound B is citric acid.